R - Change colors of specific bars in a dodged barplot in ggplot2

I'd like to change the colour of a specific date in a dodged bar chart.

The code to replicate the existing chart is as follows:

df <- data.frame(mat = c("Apr-24", "Nov-24", "Apr-25", "Nov-25", "Apr-26", "Sep-26", "Apr-27"),
             term_rv = c(0.14, 0.34, 0.19, 0.22, 0.34, 0.16, 0.27),
             zc_rv = c(0.12, 0.31, 0.23, 0.27, 0.36, 0.09, 0.29))

df2 <- gather(df, event, total, term_rv:zc_rv)

ggplot(df2, aes(x=mat, y=total, fill=event)) + 
  geom_bar(width = 0.8, stat = "identity", position = 'dodge') +
  theme_bw() +
  scale_fill_manual(name = c("Method"), 
                label = c("TF", "ZC"), 
                values = c("#ff8080", "#00cccc"))

This should produce a chart like this:

I'd like the two bars associated with Nov-24 to be in a different colour (or with borders) to differentiate them with the rest. So something like this:

Is there a simple way to do this? Feel free to change the code in anyway. I used gather to create the dodged chart but there might be a easier way.

I typically use the gghighlight package for this type of task, e.g.

library(tidyverse)
library(gghighlight)

df <- data.frame(mat = c("Apr-24", "Nov-24", "Apr-25", "Nov-25", "Apr-26", "Sep-26", "Apr-27"),
                 term_rv = c(0.14, 0.34, 0.19, 0.22, 0.34, 0.16, 0.27),
                 zc_rv = c(0.12, 0.31, 0.23, 0.27, 0.36, 0.09, 0.29))

df2 <- df %>%
  pivot_longer(cols = -mat,
               names_to = "event",
               values_to = "total")

ggplot(df2, aes(x=mat, y=total, fill=event)) + 
  geom_bar(width = 0.8, stat = "identity", position = 'dodge') +
  theme_bw() +
  scale_fill_manual(name = c("Method"), 
                    label = c("TF", "ZC"), 
                    values = c("#ff8080", "#00cccc")) +
  gghighlight(mat == "Nov-24")
#> label_key: mat

<!-- -->

If you use the 'darker colours' for all of the bars, then replace the unhighlighted bars with your 'regular colours', you can get your desired outcome:

ggplot(df2, aes(x=mat, y=total, fill=event)) + 
  geom_bar(width = 0.8, stat = "identity", position = 'dodge') +
  theme_bw() +
  scale_fill_manual(name = c("Method"), 
                    label = c("TF", "ZC"), 
                    values = c("#870017", "#3E4EC8")) +
  gghighlight(mat == "Nov-24",
              unhighlighted_params = list(fill = rep(c("#ff8080", "#00cccc"),
                                                     length(unique(df2$mat)))))
#> label_key: mat

<!-- -->

^{Created on 2023-06-29 with reprex v2.0.2}

A potential downside to this approach is the legend is lost. One potential workaround for this is to label the bars of interest, e.g.

library(tidyverse)
library(gghighlight)
library(ggrepel)

df <- data.frame(mat = c("Apr-24", "Nov-24", "Apr-25", "Nov-25", "Apr-26", "Sep-26", "Apr-27"),
                 term_rv = c(0.14, 0.34, 0.19, 0.22, 0.34, 0.16, 0.27),
                 zc_rv = c(0.12, 0.31, 0.23, 0.27, 0.36, 0.09, 0.29))

df2 <- df %>%
  pivot_longer(cols = -mat,
               names_to = "event",
               values_to = "total")

ggplot(df2, aes(x=mat, y=total, fill=event)) + 
  geom_bar(width = 0.8, stat = "identity", position = 'dodge') +
  theme_bw() +
  scale_fill_manual(name = c("Method"), 
                    label = c("TF", "ZC"), 
                    values = c("#ff8080", "#00cccc")) +
  gghighlight(mat == "Nov-24") +
  geom_text_repel(aes(label = event), 
            position = position_dodge(width = 0.8),
            min.segment.length = 0)
#> label_key: mat

<!-- -->

^{Created on 2023-06-29 with reprex v2.0.2}

An alternative approach is to create a new variable distinguishing the elements of interest and highlighting them with their own fill colours. The highlighted elements can be included in the legend if required.

library(ggplot2)
library(tidyr)
library(dplyr)

df1 <- data.frame(mat = c("Apr-24", "Nov-24", "Apr-25", "Nov-25", "Apr-26", "Sep-26", "Apr-27"),
                 term_rv = c(0.14, 0.34, 0.19, 0.22, 0.34, 0.16, 0.27),
                 zc_rv = c(0.12, 0.31, 0.23, 0.27, 0.36, 0.09, 0.29))

df2 <- 
  df1  |> 
  pivot_longer(cols = -mat,
               names_to = "event",
               values_to = "total") |> 
  mutate(event_grp = ifelse(mat == "Nov-24", paste0(event, 1), event))

ggplot(df2, aes(x=mat, y=total, fill=event_grp)) + 
  geom_bar(width = 0.8, stat = "identity", position = 'dodge') +
  theme_bw() +
  scale_fill_manual(name = c("Method"),
                    breaks = c("term_rv", "zc_rv", "term_rv1", "zc_rv1"),
                    label = c("TF", "ZC", "TF*", "ZC*"), 
                    values = c("#ff8080", "#00cccc", "#870017", "#3E4EC8")) +
  theme(legend.position = "bottom")

<!-- -->

^{Created on 2023-06-29 with reprex v2.0.2}